Process for the preparation of esters of leukotriene A

ABSTRACT

Esters of leukotriene A are conveniently prepared by reacting 9-oxo-5,6-trans-epoxynon-7(E)-enoates with 11-triphenylphosphoranylidene undec-6(Z),9(Z)-diene. Esters of leukotriene-A are useful intermediates in the synthesis of naturally occurring leukotrienes, which have been shown to be potent broncho-constricting substances. Novel intermediates to the esters of leukotriene A, 9-oxo-5,6-transepoxynon-7(E)-enoates and 7-oxo-5,6-transepoxyheptanoates, are disclosed.

BACKGROUND OF THE INVENTION

"Slow Reacting Substance of Anaphylaxis" (SRS-A) has been shown to be ahighly potent broncho-constricting substance which is released primarilyfrom mast cells and basophils on antigenic challenge. SRS-A has beenproposed as a primary mediator in human asthma. SRS-A, in addition toits pronounced effects on lung tissue, also produces permeabilitychanges in skin and may be involved in acute cutaneous allergicreactions. Further, SRS-A has been shown to effect depression ofventricular contraction and potentiation of cardiovascular effects ofhistamine.

The recent discovery of the naturally occurring leukotrienes and theirrelationship to SRS-A has reinforced interest in SRS-A and otherarachidonate metabolites. SRS-A derived from mouse, rat, guinea pig andman have all been characterized as mixtures of leukotriene-C₄,represented below as structural formula (LT-C₄) and leukotriene-D₄,represented below as structural formula (LT-D₄). ##STR1##

Leukotriene-A, represented below by the structural formula (LT-A), isbelieved to play a central role in natural conversion of arachidonicacid to the leukotrienes C₄ and D₄. ##STR2##

Leukotriene A has been converted to leukotrienes C₄ and D₄ via methodswell known in the art.

Synthetic leukotrienes have shown the same biological activities oftheir naturally occurring counterparts.

SUMMARY OF THE INVENTION

The esters of leukotriene A of this invention may be convenientlyprepared by reacting 9-oxo-5,6-transepoxynon-7(E)-enoate esters with11-triphenylphosphoranylidene undec-6(Z), 9(Z)-diene. This inventionalso relates to novel intermediates employed in the preparation of theesters of leukotriene A. These intermediates are9-oxo-5,6-trans-epoxynon-7(E)-enoates and7-oxo-5,6-transepoxyheptanoates.

DETAILED DESCRIPTION OF THE INVENTION

A process for the preparation of esters of leukotriene A, represented bythe following structural formula (I): ##STR3## wherein R is alkyl of 1to 4 carbon atoms, benzyl or 1,1,1,-trichloroethyl, comprises reacting9-oxo-5,6-transepoxynon-7(E)-enoate ester, represented by the followingstructural formula (II): ##STR4## wherein R is defined above, with11-triphenylphosphoranylidene undec-6(Z),9(Z)-diene, represented by thefollowing structural formula (III): ##STR5## Particular alkyl esters ofleukotrine A include those compounds of the formula (I) wherein R isethyl, n-propyl, i-propyl, n-butyl, i-butyl, sec-butyl, tert-butyl andespecially methyl.

The reaction of 9-oxo-5,6-trans-epoxynon-7(E)enoate ester with11-triphenylphosphoranylidene undec-6(Z),9(Z)-diene is usually carriedout at a temperature of about -78° C. to about room temperature for fromabout 10 minutes to about 12 hours. The reaction is generally run in anaprotic organic solvent which is inert to the reactants under thereaction conditions. The addition of lithium iodide to the reactionmixture may increase the amount of the desired Z isomer about the C₉double bond of the esters of leukotriene A prepared via the claimedprocess; however, even in the absence of lithium iodide the predominantstereoisomer obtained via the claimed process is the desired isomer.

The starting material 9-oxo-5,6-trans-epoxynon-7(E)-enoate ester of theformula (II) wherein R is alkyl of 1 to 4 carbon atoms, benzyl or1,1,1,-trichloroethyl is conveniently prepared from7-oxo-5,6-trans-epoxyheptanoate ester, represented by the followingstructural formula (IV): ##STR6## wherein R is defined above, andformylmethylenetriphenylphosphorane. This reaction is conducted understandard Wittig reaction conditions which are known to the skilledartisan.

In fact, the starting material 9-oxo-5,6-trans-epoxynon-7(E)-enoate (II)may be prepared in three steps from 4-formylbutyrate derivatives via thefollowing synthetic scheme: ##STR7## wherein R is described above.

The mixture of 4-formylbutyrate derivative andformylmethylenetriphenylphosphorne is refluxed in toluene to afford7-oxo-hept-5(Z)-enoate (V). Epoxidation of the 7-oxo-hept-5(Z)-enoate(V) is carried out in basic media to give 7-oxo-5,6-transepoxyheptanoate(IV), which is converted to 9-oxo-5,6-trans-epoxynon-7(E)-enoate (II).

The starting material triphenylphosphoranylidene undec-6(Z),9(Z)-diene(III) is conveniently prepared from [undec-6(Z),9(Z)-diene]triphenylphosphonium halide, represented by the following structuralformula (VI): ##STR8## wherein Hal is bromide, chloride or iodide, inthe presence of a non-nucleophilic base and an aprotic organic solvent.The conversion of the phosphonium halide (VI) to the phosphoranylidene(III) may be done in situ, that is the phosphoranylidene (III) may beformed in the reaction mixture prior or subsequent to the addition ofthe 9-oxo-5,6-trans-epoxynon-7(E)-enoate (II).

Examples of the non-nucleophilic bases which may be employed to convertthe phosphonium halide (VI) to the phosphoranylidene (III) includen-butyllithium, methyllithium, tert.-butyllithium, lithiumdiisopropylamide and sodium hexamethylsilylamide [NaN(Si(CH₃)₃)₂ ].Illustrative of the aprotic organic solvents which may be present duringthis conversion are tetrahydrofuran, hexamethylphosphoramide, tolueneand diethylether. The conversion is usually carried out at moderate tolow temperatures, especially temperature at or below ambienttemperatures.

In fact, the starting material triphenylphosphoranylideneundec-6(Z),9(Z)-diene (III) was prepared from oct-2-yne-1-ol via thefollowing synthetic route: ##STR9##

Oct-2-yne-1-ol was brominated with phosphorous tribromide and theresulting halide was coupled with dimagnesium halide salt of propargylalcohol to yield the diynol (VII). Catalytic reduction of the diynol(VII) over palladium-barium sulfate catalyst afforded exclusively theZ,Z dienol (VIII) which was converted to the allylic bromide withphosphorous tribromide. The allylic bromide was reacted with triphenylphosphine to give [undec-6(Z),9(Z)-diene]triphenylphosphonium bromidewhich was converted to triphenylphosphoranylidene undec-6(Z),9(Z)-diene(III).

The esters of leukotriene A prepared according to this invention may beemployed as intermediates in the synthesis of the naturally occuringleukotrienes. The naturally occuring leukotrienes have been shown to bepotent broncho-constricting substances and are useful as biologicalstandards against which to test potential therapeutic agents.

The following examples illustrate the preparation of starting materialsfor the claimed process and the claimed process. As such, these examplesare not to be considered as limiting the invention set forth in theclaims appended hereto.

EXAMPLE I Preparation of Methyl-9-oxo-5,6-trans-epoxynon-7(E)-enoate(Compound II, wherein R is methyl)

(a) Methyl-7-oxo-hept-5(Z)-enoate (V-R═CH₃)

A mixture of methyl-4-formylbutyrate (1.30 g, 10 mmol),formylmethylenetriphenylphosphorane (3.05 g, 10 mmol) and toluene (75ml) was refluxed for 24 hours. The reaction mixture was concentrated atreduced pressure to remove toluene and hexane was added. The reactionmixture was then filtered and the filtrate concentrated at reducedpressure to yield the crude product. The crude product was distilled atreduced pressure to afford the desired material as a pale yellow oil (bp64°-74° C. at 0.15 torr).

(b) Methyl-7-oxo-5,6-trans-epoxyheptanoate (IV, R═CH₃)

To a mechanical stirred solution of 30 percent hydrogen peroxide (8.24ml, 1.1 equiv) in water (100 ml) and methanol (200 ml) with 1 N sodiumbicarbonate (13.2 ml, 0.2 equiv) was added dropwise at ambienttemperature over one hour methyl-7-oxo-hept-5(Z)enoate (10.31 g, 66mmol) in methanol (100 ml). The pH of the reaction mixture wasmaintained between 9.25 and 9.5 by addition of 1 N sodium bicarbonate.The reaction mixture was heated to 40° C. and stirred for 3 hours. Tothe reaction mixture was added saturated aqueous ammonium sulfatesolution (500 ml) and the methanol removed at reduced pressure. Theresultant aqueous solution was extracted with methylene chloride and theextract dried over anhydrous magnesium sulfate. The resultant solutionwas chromatographed on silica gel eluting with 3% acetone/methylenechloride to afford the desired product as an oil.

    ______________________________________                                        Analysis          C      H                                                    ______________________________________                                        Calculated        55.81  7.02                                                 Found             55.65  7.29                                                 ______________________________________                                    

(c) Methyl-9-oxo-5,6-trans-epoxynon-7(E)enoate

A mixture of methyl-7-oxo-5,6-trans-epoxyheptanoate (4.8 g, 2.78 mmol),formylmethylenetriphenylphosphorane (8.93 g, 2.94 mmol) and toluene (400ml) was heated to reflux for 5 hours and allowed to stand at ambienttemperature for about 48 hours. The reaction mixture was concentrated atreduced pressure to remove toluene. The concentrate was triturated withdiethyl ether and then chromatographed on silica gel eluting with 3%acetone/chloroform to afford a yellow oil.

    ______________________________________                                        Analysis          C      H                                                    ______________________________________                                        Calculated        60.59  7.12                                                 Found             60.80  7.15                                                 ______________________________________                                    

Satisfactory spectral data (¹ H nmr, ms, ir and where appropriate ¹³ Cnmr) were obtained for all intermediates.

Similarly, the other 9-oxo-5,6-trans-epoxynon-7(E)-enoates of theinstant invention may be prepared employing analogous starting materialsunder analogous reaction conditions.

EXAMPLE II Preparation of triphenylphosphoranylideneundec-6(Z),9(Z)-diene (Compound III)

(a) Oct-2-yne-1-bromide

To a solution of oct-2-yn-1-ol (12 g, 96 mmol) in diethyl ether (40 ml)and pyridine (0.48 g, 6 mmol) was added dropwise at about -15° C.phosphorous tribromide (8.6 g, 32 mmol). The reaction mixture wasallowed to warm to ambient temperature over 1.5 hours and heated toreflux for 30 minutes. The reaction mixture was then poured ontoice-sodium chloride mixture and the organic phase separated from theaqueous phase. The aqueous phase was extracted with diethyl ether. Thecombined organic phases were dried over anhydrous magnesium sulfate andthen concentrated at reduced pressure to a slightly yellow liquid. Theyellow liquid was distilled under reduced pressure to afford the desiredproduct (bp 108°-118° C., 15 torr.)

(b) Undec-3,6-diyn-1-ol (VII)

To a slurry of magnesium metal (4.2 g, 175 mmol) in tetrahydrofuran (40ml) at 0° C. over 30 minutes was added ethylbromide (21.2 g, 194 mmol)in tetrahydrofuran (20 ml). The mixture was stirred for an additional 30minutes and propargyl alcohol (4.98 g, 89 mmol) in tetrahydrofuran (5ml) was added over 30 minutes at 0° C. The resultant Grignard reagentwas stirred at ambient temperature for 2 hours and then cooled to 0° C.Cuprous chloride (200 mg) was added and the mixture stirred for 20minutes. To this mixture was added dropwise over 30 minutesoct-2-yn-1-bromide (10.5 g, 55 mmol) in tetrahydrofuran (15 ml). Thereaction mixture was refluxed gently for 20 hours and an additionalcuprous chloride (100 mg) was added. The reaction mixture was refluxedfor an additional 5 hours, cooled to 0° C. and poured onto 2.5 Nsulfuric acid (80 ml) and ice (100 ml). The resultant mixture wasextracted with diethyl ether and the extract washed with 5% sodiumcarbonate and dried over anhydrous sodium sulfate. The resultantmaterial was distilled at reduced pressure to afford the desired productas a yellowish liquid (bp 77°-96° 0.06-0.1 torr.)

(c) Undec-3(Z),6(Z)-dien-1-ol (VIII)

A slurry of 5% palladium-barium sulfate catalyst (115 mg) in hexane (20ml) and quinoline (0.16 ml) is prehydrogenated at 35 psi of hydrogen ina Parr hydrogenation apparatus. Undec-3,6-diyn-1-ol (1.64 g, 10 mmol)was added to the prehydrogenated catalyst slurry and the resultantmixture hydrogenated for about 30 minutes. The resultant reactionmixture was filtered to remove the catalyst and the filtrate dilutedwith hexane. This solution was washed with 3 N hydrochloric acid, waterand 5% sodium bicarbonate and dried over anhydrous sodium sulfate. Theresultant material was chromatographed over silica gel and eluted withmethylene chloride to yield the desired product. The stereochemistry ofthis product was confirmed by the use of Eu(fod)₃ shift reagent.

(d) 11-Bromo-undec-6(Z),9(Z)-diene

To a solution of undec-3(Z),6(Z)-dien-1-ol (100 mg, 0.6 mmol) inpetroleum ether (5 ml) was added dropwise over 10 minutes at -20° C.phosphorous tribromide (275 mg, 10.1 mmol) in petroleum ether (3 ml).The reaction mixture was stirred for two hours at about -20° C., and onehour at room temperature and let stand for about 20 hours. The reactionmixture was then heated to reflux for 11/2 hours, cooled and poured ontoice (30 ml) and 5% sodium bicarbonate (10 ml). The organic layer wasseparated and the aqueous layer extracted with diethyl ether. Thecombined organic phases were washed with 5% sodium carbonate, water and1 N hydrochloric acid, dried over anhydrous sodium sulfate and filtered.The filtrate was concentrated at reduced pressure to afford the desiredproduct.

(e) [Undec-6(Z),9(Z)-diene] triphenylphosphonium bromide (VI, Hal═Br)

To a solution of 11-bromo-undec-6(Z),9(Z)-diene (390 mg, 1.69 mmol) intoluene (4 ml) was added dropwise at 0° C. a solution oftriphenylphosphine (1.8 g, 6.8 mmol) in toluene (4 ml). The reactionmixture is allowed to stand at 0° C. for about 20 hours and the reactionproduct precipitates out of solution. The reaction mixture is filteredand the filter cake washed with toluene and then dried in vacuo to yieldthe desired product (mp 140.5°-142° C.). Triphenylphosphoranylideneundec-6(Z),9(Z) diene is prepared by treating the above product with anon-nucleophilic base.

Satisfactory spectra data (¹ H nmr, ms, ir and where appropriate ¹³ Cnmr) were obtained for all intermediates.

EXAMPLE III Preparation of Methyl 5,6-oxido-7,9,11,14-eicosatetraenoate(Leukotriene A - methyl ester - Compound (I) R═CH₃)

(a) Triphenylphosphoranylidene-undec 6(Z),9(Z)-diene

To a slurry of [undec-6(Z),9(Z)-diene] triphenylphosphonium bromide(5.25 g, 10.6 mmol) in tetrahydrofuran (150 ml) under anhydrousconditions at -78° C. was added dropwise n-butyllithium (4.6 ml-2.3 M).Upon the completion of the n-butyllithium addition the reaction mixturewas warmed to -20° C. and stirred for 20 minutes to form thephosphoranylidene in solution.

(b) Leukotriene A - methyl ester from III(a)

To the phosphoranylidene formed according to III(a) at -78° C. was addeddropwise over a few minutes methyl-9-oxo-5,6-trans-epoxynon-7(E)-enoate(2.61 g, 12.3 mmol) in tetrahydrofuran (15 ml). The reaction mixture wasstirred for 10 minutes at -78° C., 20 minutes at -20° C. and 10 minutesat room temperature. The tetrahydrofuran was removed at reduced pressureand the residual oil is triturated with hexane. The hexane decantate isconcentrated at reduced pressure, filtered and diluted to exactly 100 mlin hexane. Ultraviolet spectra indicates product obtained in good yieldand purity.

(c) Leukotriene A methyl ester from phosphonium bromide II(e)

To a slurry of [undec-6(Z),9(Z)-diene]triphenylphosphonium bromide (200mg, 0.4 mmol) in tetrahydrofuran (3 ml) under anhydrous conditions at-78° C. was added dropwise n-butyllithium (165 μl, 0.4 mmol). Thereaction mixture was stirred at -78° C. for 45 minutes and a smallamount of the salt remains. To the reaction mixture was added at -78° C.was added dropwise methyl-9-oxo-5,6-trans-epoxynon-7(E)-enoate (103 mg,0.49 mmol). The reaction mixture is stirred at -78° C. for 30 minutesand room temperature for 15 minutes. The tetrahydrofuran was removed atreduced pressure and the residual oil triturated with hexane. The hexanedecantate is washed with water and dried over anhydrous sodium. Thedesired leukotriene A methyl ester was isolated as a pale yellow oil andwas characterized as follows:

uv(hexane) 270s, 280, 290s; mass spectrum, m/e 332.2337 (calc'd for C₂₁H₃₂ O₃,332.2351); ¹ H nmr (CDCl₃) 0.9 (t, C₄ H₈ CH₃), 1.3 (m, (CH₂)₃+CH₂ CH₂ CO₂ Me), ##STR10## 2.35 (broad t, CH₂ CO₂), ##STR11## ═C--CH₂--C═), ##STR12## 3.66 (s, OCH₃), 5.45 (m, CH₂ -HC═CH-CH₂), ca. 6.3(CH═CH₃); ¹³ C nmr (CDCl₃)ppm 14.0 (C₂₀), 21.4 (C₂), 22.5 (C₁₉), 26.(C₁₇), 27.3, 28.0 (C₁₃), 29.3 (C₁₇), 31.3, 31.5 (C₁₈, C₃), 33.6 (C₄)51.5 (CH₃ O), 58.3 (C₆), 60.5 (C₅), 174 (C₁), 124-135 (vinyl). Thecomplexity of the vinyl region as well as the observation of doublets inthe ¹³ C spectrum for both C₁₃ and C₆ suggests that the final productwas a mixture of E and Z isomers about the C₉ double bond.

What is claimed is:
 1. A process for the preparation of alkyl esters ofleukotriene A, represented by the following structural formula (I):##STR13## wherein R is alkyl of 1 to 4 carbon atoms, benzyl or1,1,1-trichloroethyl, which comprises:(a) reacting7-oxo-5,6-trans-epoxyheptanoate ester represented by the followingstructural formula (IV) ##STR14## wherein R is defined above andformylmethylenetriphenylphosphorane to afford9-oxo-5,6-trans-epoxynon-7(E)enoate ester, represented by the followingstructural formula (II) ##STR15## wherein R is defined above; and (b)reacting 9-oxo-5,6-trans-epoxynon-7(E)enoate ester withtriphenylphosphoranylidene undec-6(Z),9(Z)-diene, represented by thefollowing structural formula (III) ##STR16##
 2. A process according toclaim 1 wherein R is methyl.
 3. A process according to claim 1 whereintriphenylphosphoranylidene undec-6(Z),9(Z)-diene of the formula (III) isprepared from [undec-6(Z),9(Z)-diene]triphenylphosphonium haliderepresented by the following structural formula (VI): ##STR17## whereinHal is bromide, chloride or iodide, in the presence of anon-nucleophilic base and an aprotic organic solvent.
 4. A processaccording to claim 3 wherein triphenylphosphoranylideneundec-6(Z),9(Z)-diene of the formula (III) is prepared in situ.
 5. Aprocess according to claim 3 wherein the non-nucleophilic base isselected from the group consisting of n-butyllithium, methyllithium,tert.-butyllithium, lithium diisopropylamide and sodiumhexamethylsilylamide.
 6. A process according to claim 3 wherein theaprotic organic solvent is selected from the group consisting oftetrahydrofuran, hexamethylphosphoramide, toluene and diethyl ether. 7.A process according to claim 1 wherein the reaction is carried out at atemperature of about -78° C. to about room temperature for from about 10minutes to about 12 hours.